1. Field of the Invention
This invention relates in general to a switch device and in particular to a switch device that carries out an opening and closing action in conformity with variations in temperature.
2. Brief Description of the Prior Art
Electrical appliances such as motors generate heat when they are subjected to an overload and if they are continuously used despite such overload the insulating coating of the coil is burnt, thereby demaging the motor and making it impossible for them to function any longer. In order to prevent this it is conventional to use a switch device (protective device) that employs a bimetal which opens a movable contact due to abnormal electric current and abnormal temperature, thereby bringing about the OFF state.
Examples of the above-described switch device are shown in FIGS. 9 through 11.
In the switch device 41 which is shown in FIG. 9, a bimetal disc 7 is fixed to a bolt 5 via a washer 30. This fixing is effected by deforming the head 5c of the bolt 5 (or by screwing the same). The bolt 5 is screwed to a female screw 5b which extends through a bore in bottom wall 3 of electrically insulative housing 2 and is secured to the bottom wall 3 as it is tightened by means of a lock nut 6. In the insulative housing 2, terminals 13b and 14b of the fixed contact point 13a and 14a are fixed as by staking and the insulative housing 2 is covered by an electrically insulative lid 16.
At the time when the motor is in normal operation, the movable contacts 8 and 8 which are mounted on the bimetal disc 7 are in engagement with the stationary contacts 13a and 14a and the terminals 13B and 14B that protrude under the insulative housing 2 are electrically connected through the stationary contacts 13a and 14a, movable contacts 8 and 8 and bimetal disc 7. Switch device 41 is installed at a common terminal of the motor and, if the motor happens to be in an abnormal or overload state, an abnormal electric current flows to the common terminal, with a result that the bimetal generates heat (Joule's law), is heated and snaps open with the movable contacts 8 and 8 moving out of engagement with the stationary contacts 13a and 14a, thereby electrically deenergizing terminals 13b and 14b.
The interior of the switch device 41 is heated to a suitable temperature by means of a heater 15a so that the deformation or snapping of the bimetal disc 7 at the time of a fault condition may take place early. In the drawing, 15b is the terminal of the heater 15a. In this manner, the motor is maintained in an acceptable temperature range.
In a switch device 42 shown in plan view in FIG. 10 and in FIG. 11 which is a cross section taken along line 11--11 of FIG. 10, there is located a can 26 electrically isolated from a metal plate 22 via an insulation sheet 24. Metal plate 22 and the can are fixed mutually in such a manner that the lower portion 26a of the can 26 may clamp the metal plate 22 via an electrically insulative gasket 29. A stationary contact 23a is mounted on plate 22 and a terminal 23b is formed thereon. One end of the bimetal strip 27 is fixed to the inner wall of the can 26.
In the drawing, numeral 31 stands for a rivet head that mounts a bimetal strip 27 and its shank 31a is inserted through an aperture in the can 26 and welded to the can. As a result, the bimetal strip 27 is firmly fixed to the can 26. A heater 25a is wound on the peripheral surface of the can 26 and one terminal 25c of the heater 25a is connected with the can 26 by means of welding.
At the time when the motor is functioning normally, a movable contact 28 that is provided at the other end of the bimetal strip 27 is in contact with the stationary contact 23a and the terminal 23b and the terminal 25b of the heater 25a are electrically connected via the metal plate 22, stationary contact 23a, movable contact 28, bimetal strip 27, can 26 and the heater 25a.
The method for using the switch device 42 and the principle governing its action are the same as those of the switch device 41 described in FIG. 9.
In the case of a comparatively large-sized motor of more than one half HP, for example, the switch device 41 or 42 is electrically connected with the common terminal of the motor and is ordinarily used as shown in FIGS. 8a and 8b.
At the time when the motor is functioning normally, the contacts of the switch device are in engagement conducting the operating current. At the time when the motor load is excessive, however, the overload electric current is detected and the circuit is opened.
At the time when the motor experiences some trouble and the rotary element is locked, thereby causing an electric current which is several times as large as the normal operating current to flow, the switch device responds to the said abnormal electric current, with a result that the circuit is opened.
In the case of an overload state and a fault such as the locked state, etc., the switch device opens the electric circuit, thereby protecting the motor from being burnt. The temperature setting of the bimetal is so arranged that an automatic return of the switch device may be effected, with a result that the OFF/ON is repeated until the abnormal state is removed.
Since there is a limit to the life of the switch device, however, there are cases where the abnormal condition may not be removed indefinitely. In such a case, the switch may fail in the contacts closed position and the abnormal current will be passed continuously to the motor, with a result that the motor may be burnt or set on fire.